Instrument including means for determining the horizontal direction



March 12, 1929. w, w sou 1,705,146

INSTRUMENT INCLUDING MEANS FOR DETERMINING THE HORIZONTAL DIRECTION Filed June 11, 1925 2 sho etysheet l ar/Q [raven Zor fiaberl Mela-71671502 deci March 12, 1929; R. w. WILLSON INSTRUIBNT INCLUDING MEANS FOR DETERMINING THE HORIZONTAL DIRBCTIOF Filed June 11, 1925 2 Sheets-Sheet 3 izvenior RoberiWfieeZerM/Zs flriizuzii 52-00 on, decd Z:s,' admr Patented Mar. 12, 1929.

UNITED. STATES PATENT OFFICE.

ROBERT WHEELER WILLSON, DECEASED, LATE OF CAMBRIDGE, MASSACHUSETTS, BY

ARTHUR H. BROOKS, ADMINISTRATOR, OF CAMBRIDGE, MASSACHUSETTS, AS- SIGNOR, BY MESNE ASSIGNMENTS, TO BRANDIS & SONS INC., 01' BROOKLYN, NEW

YORK, A CORPORATION OF NEW YORK.

INSTRUMENT INCLUDING MEANS FOR DETERMINING THE HORIZONTAL DIRECTION.

Application filed June 11, 1925. Serial No. 86,384.

This invention relates to improved instruments for determining the horizontal direction in relation to a point of observation.

Instruments of the class improved may or 5 may not also include means for observing,

measuring or relating the direction of a terrestrial or celestial object or point to the horizontal plane, or means for measuring an angle between the object and such a plane, or

between objects in such a plane.

The invention is adapted to use in any situation in which the determination accurately of the relation of an thing'to the horizontal direction is desira is or fundamental, such, for instance, as in levels for machine beds, sighting levels for constructorsand engineers; artificial horizons; surveyors levels, theodolites, transits, plane-tables and compasses; mariners and air-pilots sextants and quadrants, compasses and gy'roscopic compasses; astronomical instruments such as sun-dials, astrolabes, or equatorial, altazimuth, meridian-transit and zenith-transit telescope mountings; mapping, measuring andreoording devices employing a hobo.-

graphic image of the object field, an precision cameras in general; to levels associated with instruments such as chemists balances, galvanometers and seismographs; and the invention therefore includes instrumental levels useful in places in which the relationship of a point or thing to the geodetic vertical or the horizontal derived therefrom is important. The invention is particularly ap plicable to situations in which means for determining the horizontal or vertical by observation of the natural horizon, or a plumbline or such device is impracticable or difiicult, as for navigation at night or in thick 40 weather; and especially for the navigation of air-ships and air-planes.-

Principal objects of the invention are to provide an instrument for the above and other uses which shall give a delicateand trustworthy indication of the horizontal in rela-' tion to the direction ofgravity atthe point of observation; to provide instrumental means for this purpose capable of association with observing and measuring instruments; to pmvide optical means for accurately de- .terinining the relative position of a distant object in respect to the horizontal, independently, within limits, of the actual position of or motion of the observing instrument; and to provide means in such an instrument for rendering the observation direct, simple and capable of being made by relatively unskilled persons.

For these and other purposes the invention includes an improved type of liquid-and-bubble level in association with optical devices establishing a datum-point of little angular magnitude moving with and representing the true position of the bubble, and therefore capable of indicating the position of the level members with increased accuracy; and provides for reading the indication of the level plane by optical comparison either directly with a distant ob'ect or if desired, by measured angular re ection orreflections. The invention also includes appro riate auxiliaries for the adjustment of, tor mounting and for illuminating its operative parts.

The invention will now be explained for particular specific instances, in connection with sighting levels for miscellaneous uses, for example as a mariners or air-pilots sextant, in which the horizontal datum-plane may be obtained by the artificial indication of the instrumental level instead of or alternatively with direct observation of the natural horizon. But it will be understood that the means for indicatin itself capable of the a eve-mentioned and other uses as well, and that the specific in- 'vention in sextants is a species only of the genus constituting the invention.

In the accompanyin drawings:

Fi 1 is :1. Ion itudinal section through a simp e'sighting evel embodying a feature of the invention;

Fig. 2 is a similar section illustrating the adaptability of features of the mventlon to telescopic levels;

the horizontal is in Fig. 3 is a longitudinal section through the optical elements of a sextant according to the invention;

Fi 4 is a longitudinal section through the hand e of a sextant showing a mode of housing a primary battery for illuminating the level-bubble of such a sextant;

Fig. 5 is an enlarged dia ram in lon tudinal section of the level-indicatin to escope shown in Fig. 3, illustrative of t e optical properties 0 this part of the instrument;

Fi 6 is a similar di ram of a bubble level illustrating its opti properties 5 Figs. 7, 8 and 9 are diagrams similar to Fi 5 showing a modification; and

Figs. 10 and 11 are diagrams of visual appearances of telescopic images.

Referring now to the drawings, it is preferred to em loy as an indicator of the horizontal direction a bubble or spirit level 1 of which a preferred type is illustrated in the comprising a concave meniscus glass 2, the under surface of which is preferably a ent of a sphere, beneath the underside of which a pool of a lim id fluid 3 is maintained b the remaining w s of a cell comprising a p ane-parallel piece of lass 4 and a suitable spacin annulus 5, whic may be also of glass groun to fit the mar 'n of the concave face of the meniscus 2. e plane-parallel plate 4, annulus 5 and the meniscus 2 may be held together in any suitable way, preferably by a strong cement such as a viscous solution of water glass, or by fusion, or a fusible cement. The fluid 3 does not quite fill the cell 2, 4:, 5, which contains an air or vapor bubble B to serve as the index.

It is preferred that the fluid 3 shall have an index of refraction of 1.5 or thereabouts in order that parallel light striking the bubble B in a generally vertical direction may be refracted by the virtual meniscus lens made up of the intervening toroid film and p, Fig. 6, which the viscosity and sur ace tension of the fluid cause at all times to be retained between the bubble B and the lower surface of the meniscus 2. If the index is about 1.5, parallel light actin through the virtual meniscus p willbe ro htto an emergent pencil c, 'g. fikgreatly er than the bubble, and lyin in e mean center of figure of the lower sur ace of the bubble. This little pencil of light is intensely brilliant when the index is viewed from beneath; under proper conditions of illumination the body of the bubble B appears from below as a dark shadow b with a central spot of light e, Fig. 10, re resenting the rays 6, occupying its center. xcept transmission losses, all of the li ht which would have been intercepted by t e whole bubble is emitted through the spot a in the parallel rays e. When the illumination of the bubble B is from a wide angle of convergent light, the bubble appears as dark ring b, Fig. 11, concentric with the true central position of the bubble. These central illuminations of the round image of the round or spheroid bubble constitute a fiducial point reliable to indicate the degree of axial deviation of the bubble when the axis of concave 2 is not vertical.

A fluid 3 having the desired limpidity and low solidification point, the desired qualities of surface tension and particularly the desired index of re action, is the etroleum derivative xylol but other liqui s are approximately as desirable.

Forthe purposes of so muchof the invention as described above, it is desirable to view the bubble B from beneath and to illuminate the spherical surface 2 and the bubble by parallel light, although many of the new features of this invention may be attained without these provisions, as will presentl alppear. Referrinito Fig. 9 one wa 0 il uminating the b b Is B in parallel lig t is to mount above it a lens 8 having at its principal focus a in hole aperture 9, which for the purposes 0 the invention may be wide enough to admit the desired bright illumination. The lens 8 is thus one form of collimator for the illuminating beam of light coming throu h the aperture 9. Referring to Fig. 9 it wilFbe apparent that if the level instrument 1 is mounted for free movement, the motions of the mounting of the instrument 1 through the are measured by the free under surface 2 defined by the annulus 5 cause the bubble 6 under the influence of gravity to move with respect to the instrument about a center so at the radius of curvature of the under surface of the meniscus 2. If this radius of curvature is a distance R, the angular displacement of the bubble B with respect to the point m'will measure the angular displacement of the instrument.

If the instrument mounting also includes a mirror m capable of transmitting as well as reflecting light, and mounted at an angle of 45 to the central vertical axis :0, as and to the central horizontal axis h, h of the mounting, then the apparent motions of the bubble as viewed by reflection in the mirror m from a int 0 on axis h, hwhose distance 2, m, o, is equal to the distance R, will measure the vertical angular displacement of axis h, 11..

As observed from the point '0 the a parent position of the meniscus 2 is now vertical and osculant to the vertical plane 3 1 displaced behind the intersection of the surface m with the vertical axis 0:, a2 and horizontal axis 10., h by the distance 1', equal to the axial distance 2, m. Supposing the observers eye to be at the point n, tipping the objective end of the instrument down will cause the reflection of the bubble B in m apparently to move down, and tipping the instrument up will cause the reflection o the bubble B ap arently to move up. If the princi al focal istance r of the lens L is equal to e radius R of the meniscus 2, and the distance 2, m, '0, which is the prenodal point of one of the magnifying eyepieces E, E E, E, E, of a telescope of which the line it, h is the optical axis 0 an objective L having its rincipal focus in the plane :1 y, then the rear. image of the object, P

terrestrial or celestial, from the lens L formed at plane 3 y will have the apparent motions in respect to the axis h, h through the same distances in the same directions and at the same rates as the bubble B whenever the axis h, h is moved.

By these arrangements the bubble B, wheni ever it is in the field of view by an approach to the horizontal of the axis h, h, is always in visual coincidence with any image of the true horizon through the lens L. If 'now, by reflection, the image formed by objective L of a celestial object in the vertical plane of the instrument is brought into apparent coincidence with the image of bubble B by an angular motion of a reflector away from a position causing the image of the horizon to coincide with that of the bubble, the motion of the reflector is through one-half the altitude of the celestial ob'ect. For instance, the image of a celestial o ject reflected from the index mirror 10 on the radius arm 11 and the horizon glass 12 of a sextant or other altimeter of which the observing telescope 20 is provided with one of the instruments 1 and its appurtenances may be brought into line with the bubble by superposing the image of the fiducial point and the image of the celestial object by motion of the radius arm 11, with the great advanta that both images are in the field of view an both movin whatever the unsteadmess [within the angle of view] of the observer and of the instrument. The solar image S, Fig. 10 is merely brought into concentric relation with the hubble shadow b and the fiducial point bright spot If the original position of arm 11 was such as to cause the horizon and the bubble image to coincide, the angular motion of arm in the same direction plane unsilvered glass, a level L whose meniscus surface 2 is of the radius of curvature r, 1', which is also the distance 26, 27, and a colored screen-glass 28 above the level 1. In use, the

lace level with the observers eye is the place in line with the bead sight and the center of the reflection of the bubble B which is at the optical focal distance of the head 27. The error of refractive parallax of the glass m 1s a constant error, compensated for by the set position of the head 27.

For greater accuracy and to avoid coordinatin with any mark showing the instrument axis, the telescopic form of Fig. 2 may be emloyed, comprising the telescope 30 whose obective L has a principal focus 1 on lane y, y, coincident with the focal distance 0 eye-piece E and the distance by reflection in an inclined mirror m from the nodal point '0 of the eyepiece of the bubble B in a level 1. The glasses m m may be prisms of different indices of refraction adapted to transmit an axial am without oflset parallax, as will occur when their respective refractions for the visual maximum wave-length are complementarily less and greater than the angle of deviation of incident light in respect to the position of the juncture plane of the prism, and the prisms are of the same thickness at their centers.

In the instances mentioned, the bubble level may be accurately mounted in a perpendicular branch of the sight-tube, and the transparent mirror for observing it may occupy a position across the juncture with the sight-tube.

For use in sextants, surveyors levels and theodolites, where the fiducialoint spot e in the bubble image is most use 111, an optical s stem securin parallel light transmitted t rough the bu hble is recommended, as described in connection with Fig. 9. Arrangements for this purpose may vary. As shown in Fig. 8, for example, the erect tube 35 in which the level 1 is mounted may carry a colored screen 36 and a daylight reflector 37 of white material, a hole in a diaphragm 38 being at the principal focus of a concavo convex lens 40, the concave face of which provides the meniscus surface 2 for the bubble B as above described. 1*

As shown in Fig. 7, a longer-focus concavo convex lens 41 may be similarly employed with a level 1 below the axis of the sight tube, with an illuminating colored glass screen or screens 42, 43 and replaceable diaphragm 44 above the light tube, a plane mirorr m beneath the leyel, and a plane parallel glass reflector 46, the lower face of which may be halfsllvered, at 45 to the axis .of the sight tube. The effect of this is to cause. the rays from the opening in diaphragm 44 to be collimated to parallel by lens 41, and to be reflected in this state at m. The fiducial-spot pencil c, Fig. 10, is thus reflected back through lens 41 and from the under surface of .mirror 46 to eyepiece E. The distance a, y is equal to the radius of surface 2 as before. Consideration of the figure will show that 0 tically the bubble B virtually moves in the p ane y, y. The device of the second reflection at m is useful for compactness and to accommodate a greater length of telescope and a wider angle of field of vision, and also because the relatively parallel state of the light between the reflector 4-6, the reflector m? and the bubble B permits an image by reflection from mirror 46 without a double image from the top and the bottom surfaces of mirror 46 respectively.

In the form shown in Figs. 3 and 5, preferred in some cases, particularly for sextants, the reflecting surface across the axis of the telescope 20 may be the bare or lightly-silvered hypothenuse face m of a 90 prism 50, which ma be su plemented by a second prism 51, i desired The bubble level is above the telescope and is viewed by downward reflection from reflector m in a convex mirror 52 and transmission through the prism and through the prism 51, if employed.

In each of the forms uses for night or for aeronautic navi ation imply artificial illumination of the eld behind the bubble image. As shown in Figs. 3 and 4, the handle 55 of the sextant shown in Fig. 3 may be recessed at 56 to receive a dry battery D, provided with contacts M57 and 58 to a conductor cable 59, one of these contacts being controlled by a push button switch 60, Fi 4, convenient to the observers thumb. Ca le 59 carries current for a suitable incandescent lamp 61 mounted above a colored screen 62 in the upper end of the cross tube for the level 1. As shown in Fig. 3, thefilament of the lamp may be so small as to avoid use of a diaphragm or arrangements for parallel beam illumination. Any suitable variable resistance in circuit with the lamp 61, or exchangeable diaphr s or screens may be employed for regulating the amount of the illumination.

When the screen 62 is translucent, the illumination of the bubble B is from diverse directions, and its image has the annular form nated with colored light from screen 62 with v entire ease.

The artificial horizontal indication of the bubble sextant need not prevent sextants containing it from use in the. usual way when the real horizon is not obscured. The usual horizon shade glasses 66 ma be supplemented by an opaque screen 67 or use when the to the tive an artificial horizontal is being observed. Removal of the screen 67 converts the instrument instantly to the ordinary type of sextant.

vantage of this invention is the o erative efliciency permitting an altitude ang e to be measured by coordination of two appearances only, while the observer and the instrument are oscillating through any angle less than the angle of the field of view. Bubble levels of the described type have moreover, so rapid a response to forces and so short a natural time of oscillation as to enable the relatively slow accelerations, pitchin and rolling motion of air-planes and sum 1 ships to be left out of account. There is no a preciable natural oscillation period of the ubble-cell liquid to enter as an error into the reading of the indications.

What is claimed is:

1. A liquid level, the liquid havin an index of refraction of substantially 1.5 or light of the wave-length of maximum luminosity.

2. A liquid level having a concave transparent upper wall, the li uid havingan index of refraction of substantially 1.5 for light of the wave-length of maximum luminosity.

A liquid level comprising a transparent cell containing xylol and a gaseous ubble therein.

4. In an instrument for determining the horizontal direction in relation to a point of observation the combination of a transparent liquid level containing a liquid of an index of refraction of substantially 1.5, a bubble therein, means for observing the bubble from the under side, and means for illuminating the bubble by transmitted substantially parallel light.

5. In an instrument for determinin the horizontal direction in relation to a point of observation the combination of a transparent liquid level containing xylol, a bubble therein, means for observing the bubble from the under side, and means for illuminating the pubble by transmitted substantially parallel i ht.

6. The combination with a telescope having an objective and a transparent reflector at an angle to its axis, of a bubble level ada ted to be observed in said reflector, said level having a spherical surface of a radius equal to the principal focal length of said obgective.

The combination with a telescope having an objective and a trans arent reflector at an angle to its axis, of a hub le level adapted to be observed in said reflector, said level having a spherical surface of a radius ual rincipal focal len of said ob ec- 5 an eye piece on t e o tical axis of the objective havin a nodal point distant by reflection in said re actor from the said spher- As a navigation instrument, a great adice] eurfece b a distance equal to said radius. iy same and equal to the radius of said sur- 8. The com ination in a. telescope of an obace. jective, a. bubble level having a surface of Signed b me at Boston, usetts, circular cross-section, a transparent reflector, this eighth ay of June, 1925.

6 and an eye-piece, the distance to said surface by reflection in said reflector from the- ARTHUR H. BROOKS, nodal point of the eye-piece and to the im- Administrator of Robert Wheeler Will-son, age-plane of the objective being substantiel- Deceased. 

